All electronic circuits contain capacitors. These devices store and dispense electrical charge in response to circuit currents, resulting in predictable increases and decreases of voltage across their terminals. It is this predictable and limited short-term variation of terminal voltage that makes capacitors useful as coupling and filtering devices in electronic circuits. Specifically, capacitors are useful in circuit locations where one does not want the voltage to change rapidly. One excellent use of capacitors is to minimize or "filter-out" both random and periodic fluctuations from the output stage of direct-current (DC) power supplies.
The ability of capacitors to perform their filtering function is limited by the unwanted "parasitic" resistances that result from the use of real-world, non-ideal materials in their construction. These parasitic resistances, collectively known as the finished capacitor's equivalent series resistance (ESR), manifest themselves in the user's electrical circuit as though they were a discrete resistor connected in series with the capacitance of the capacitor. Whereas ideal capacitor elements inherently oppose rapid voltage shifts in the face of changing current, the voltage across resistors changes instantaneously and proportionally to changing current. So if a practical capacitor has significant ESR, the resulting instantaneous voltage shifts across the capacitor's ESR, that are proportional to changes in circuit current, undermine the voltage-stabilizing influence of the capacitor. If the ESR becomes too large, the capacitor becomes useless as a filtering device.
Early electronic circuits designed with vacuum tubes typically employed high voltages and relatively low currents. This was a consequence of the high impedance (ratio of voltage to current) of these early electronic devices. Capacitors used in power supply filter applications typically filtered high voltages (&gt;100 V), experienced low fluctuating currents (ripple current&lt;1 A), and rarely experienced significant ripple current at frequencies greater than 120 hertz (cycles per second). Routine manufacturing methods and materials yielded filter capacitors with low enough ESR to easily satisfy the needs of these early electronic circuits.
Change has come to the world of electronics in several forms, one of which is change in the kinds of active devices employed in circuits. Vacuum tubes were replaced by discrete transistors and, now, discrete transistors have largely been displaced by integrated circuits. Even integrated circuits have evolved steadily. An example is microprocessors where the number of active devices (transistors) in a state-of-the-art microprocessor chip doubles every year or so. The most significant impact of this evolutionary change is that power supply voltages have fallen steadily (to limit power consumption in the circuits) and supply currents have risen (reflecting the greater number of active devices in integrated circuits and the higher frequencies at which these devices operate).
Since circuit voltages are now lower and currents higher, the ESR of capacitors has become of critical interest to the engineers who design these modern circuits. In contrast to the days when if a capacitor's capacitance were high enough to meet the circuits needs then it's ESR would almost certainly be low enough, today's circuits demand such low ESR that engineers frequently must use more capacitance than was formally required in order to obtain sufficiently low ESR for proper circuit operation. This leads to circuits that are physically larger and more expensive than is necessary. There is a distinct need for capacitor manufacturers to supply capacitors whose ESR is low enough to meet circuit requirements without the need to employ excessive capacitance (and occupy unnecessary circuit volume).
Another change in the world of electronics is the shift from point-to-point wiring to the use of printed circuits, and the subsequent shift from the use of leaded components on printed circuit boards to the use of surface-mounted components. These changes have provided marvelous improvements in circuit compactness and manufacturing productivity, but have had significant impact on the physical requirements of electronic components. In the case of point-to-point wiring, if a component were sensitive to soldering heat, a discrete heat-sink could be attached to the component's leadwire between the point of soldering and the body of the component to minimize component heating during the soldering process.
In the case of circuit boards used with leaded components, the leads were inserted through holes in the board and were subsequently soldered on the opposite side. This arrangement reduced the practicality of heat sinking during soldering. But at least the heat was applied on the side of the board opposite from that of the component, thus limiting component heating to that caused by heat conduction through the leadwires.
Today, almost all components are mounted to the surface of circuit boards by means of infra-red (IR) or convection heating of both the board and the components to temperatures sufficient to reflow the solder paste applied between copper pads on the circuit board and the solderable terminations of the surface mount technology (SMT) components. Not only is heat-sinking impractical in this case, it would actually defeat the effectiveness of the soldering method. A consequence of surface-mount technology is that each SMT component on the circuit board is exposed to soldering temperatures that commonly dwell above 180.degree. C. for close to a minute, typically exceed 230.degree. C., and often peak above 250.degree. C. If the materials used in the construction of capacitors are vulnerable to such high temperatures, it is not unusual to see significant positive shifts in ESR which lead to negative shifts in circuit performance. SMT reflow soldering is a significant driving force behind the need for capacitors having temperature-stable ESR.
Another driving force behind the need to manufacture capacitors having temperature-stable ESR is high circuit temperatures. As circuits are miniaturized, it becomes more difficult to remove the heat that is generated by normal circuit operation. Thus, it is not unusual for capacitors to operate in high ambient temperature environments (up to 125.degree. C.). Also, electronics are becoming an integral part of automotive applications, especially in under-the-hood applications. It is not unusual for the ambient environment to reach 150.degree. C. in such applications with the desire to go to 175.degree. C. and potentially higher. Another issue with automotive applications is temperature cycling and thermal shock. It is essential that ESR remain stable in the face of both high temperature and high rates of change of temperature.
A final threat faced by capacitors is humidity. Some high-reliability circuits are cleaned after the SMT mounting process to remove contaminants (flux residues and other contaminants). Freon-based solvents were used in the past with a high degree of cleaning effectiveness and minimal impact on component reliability. Today, with concern about the use of potentially ozone-depleting substances, many electronics manufacturers are using cleaning systems that are water-based.
Typical cleaning cycles can last for more than an hour and components can be exposed to significant heat and humidity which can cause moisture to permeate through the component's case, potentially saturating the device. Also, since much manufacturing is done in the "Pacific-Rim" countries, it is not unusual for components to be exposed to significant heat and humidity if they are stored for significant time in non-air-conditioned warehouses. Moisture can degrade ESR by attacking the integrity of electrical connections within the capacitor with a combination of oxidation and corrosion. It has become essential that capacitors resist unwanted positive shifts in ESR when they are exposed to high-humidity environments.
Production of low-ESR capacitors has proven to be a formidable challenge. Capacitors manufactured with traditional materials and methods have excessively high initial ESR. Moreover, after these capacitors are exposed to SMT reflow temperatures by the user, the ESR tends to shift upward still more. After reflow mounting, exposure to humidity, high operating temperatures, and/or thermal shock results in further, steady deterioration of ESR which can ultimately lead to the failure of the device to perform adequately in the circuit.
A particular capacitor that is desirably optimized for low ESR is a valve-metal, solid-electrolyte, surface-mount electrolytic capacitor. Such capacitors have as their dielectric thin, highly insulating anodic oxide film which may be produced electrolytically on the so-called valve metals (e.g. tantalum, aluminum, titanium, and niobium). The positive terminal of the finished capacitor is connected to the non-oxidized portion of the valve metal which supports the metal-oxide dielectric layer. The negative terminal is connected to the outermost layer of a succession of conductive layers which are formed on the exposed surface of the dielectric layer. A typical valve-metal capacitor comprises one or more metal-oxide capacitor elements which are connected electrically to metallic terminals and which are encapsulated in a protective plastic covering, coating, or case.
It is desirable to produce valve-metal, solid-electrolyte, surface mount electrolytic capacitors having low ESR. It is also desirable to produce capacitors that are substantially unaffected by heat from SMT reflow soldering, humidity exposure at elevated temperature, aggressive thermal shock conditions, and continuous operational exposure to high temperatures.